1. Field of the Invention
The invention relates to an optical scanning device comprising a radiation source for generating a radiation beam, an optical system for focusing the radiation source to a focal point on a plane of a medium, a focus actuator for shifting the radiation beam focal point relative to the plane, a focus detection unit for deriving a focus error signal which, in a predetermined focus range, is indicative of the distance between the focal point and the plane, and for deriving a focus position signal which denotes whether the focal point is positioned outside said focus range, a focus control unit comprising a control circuit for deriving an actuator control signal in response to the focus error signal so as to keep the focal point essentially in the plane, and a signal generator responsive to the focus position signal for generating a substitute focus actuator control signal that varies with time when the focus position signal denotes that focal point has transgressed the focus range and for terminating the generation of the substitute control signal in response to a signal that denotes that the focal point has come back to the focus range.
2. Description of the Related Art
A device of this type is known from U.S. Pat. No. 4,669,074. The optical detection device described therein is included in an optical reading system for reading optical record carriers having a reflective recorded information layer. During the reading operation the focal point of the radiation beam is kept in the plane of the information layer. Due to, for example, mechanical shocks or disc defects, the focal point may happen to depart from the focus range, so that there is no longer a focus error signal available and no focus control is possible any more. In such prior-art device the departure from the focus range is detected, and after such a detection the substitute control signal varying with time is generated in the form of a triangular-wave signal which cause the focal point to be moved to and fro perpendicular to the information layer. During this operation the focal point passes the focus range and the generation of the substitute control signal is terminated during the passage of the focus range. Since no information can be read from the information layer during the period of time when the focal point is outside its focus range, it is important to bring the focal point back to the plane of the information layer as quickly as possible.